To appreciate the nature of the present invention, it is helpful to first discuss the general principles of ultrasonic echoscopy (which is also known as ultrasonic echography, though strictly the two terms have different meanings). In ultrasonic echoscopy a short pulse of ultrasonic energy, typically in the 1-30 MHz frequency range, is directed into an object to be examined and any acoustic impedance discontinuities in the object reflect some of the energy. The reflected energy, or echo, is converted into an electrical signal and displayed on a cathode ray oscilloscope, a film, a chart, or in any other convenient form. This display, which provides information about the examined object to the user of the equipment, is known as an echogram.
The echogram may be either a one dimensional or a two dimensional representation. In both cases, the information is contained in the position and magnitude of the echo displayed. In a one dimensional display, the position along a base line is used to indicate the distance to the reflecting surface whilst the magnitude of the echo is displayed, for example, as a deflection of a base line or as an intensity change. In a two dimensional display, the position along a base line is used to indicate the distance to the reflecting surface as in a one dimensional display, and the direction of the base line is used to represent the direction of propagation of the acoustic energy. The two dimensional display is obtained by changing this direction of propagation of the acoustic energy and by instituting a similar, but not necessarily identical, movement of the base line of the display. The magnitude of the echo is displayed as for a one dimensional display (for example, as a deflection of the base line or as an intensity change).
The technique of ultrasonic echoscopy is used in medical diagnosis to obtain information about the anatomy of patients. The application of the technique has been described, for example, in the paper by D. E. Robinson in the "Proceedings of the Institution of Radio and Electronics Engineers, Australia", Volume 31, No. 11, pages 385-392, November, 1970, entitled "The Application of Ultrasound in Medical Diagnosis". As pointed out in that paper, ultrasonic echoscopy may be used to produce displays resembling anatomical cross-sections, and such displays have proved clinically useful when the desired information concerns the physical dimensions or the shape of organs, structures and the like. Ultrasonic echography has proved of particular value as a diagnostic aid in those areas of the body which contain soft tissue with little bone and air, particularly the abdomen and pregnant uterus, eye, breast, brain, lung, kidney, liver and heart. In general, the technique is considered to complement other techniques to provide a more complete picture of the patient's condition. However, particularly in pregnancies, ultrasonic echoscopy may be useful in place of X-rays as the latter may not give sufficient information, or may be dangerous.
Although ultrasonic echoscopy has uses other than as a diagnostic aid, this medical application of the technique provides a convenient example and will be used in the continuation of this description. In practice, a pulse of ultrasonic energy is transmitted into a patient in a known direction and echoes are received from reflecting surfaces within the body. The time delay between a transmitted pulse and the received echo depends on the distance from the transmitter to the reflecting surface and the distance information so obtained may be displayed in a suitable way for interpretation and clinical use as a one dimensional range reading or as a two dimensional cross-section as previously described. Now, when a pulse of ultrasound is propagated into any medium, echoes will be received at various time delays, which are proportional to the distances from the transducer producing the pulse to the reflecting surface if the velocity of propagation of ultrasound in the medium is constant. In soft tissues found in the human body, the velocity of sound is reasonably constant and pulsed ultrasound provides a convenient method of measuring the depth of a particular structure from the transducer face without inconvenience to the patient. This information can be used in a number of ways.
In the simplest form of display, known as "A mode", the echoes are presented as deflections of the trace of an oscilloscope, with distance being represented along the time axis. This mode is useful clinically when the source of the various echoes displayed can be positively identified. It is possible to measure the distance between two echoes, or between the energising pulse and an echo, with accuracy but it may not be possible to identify the source of the echoes. It has been used to measure the size of the baby's head inside the uterus, the depth of the eye and the bladder, and to locate the midline of the brain. Similar information may be displayed by use of the "B mode" display, which is a cross-sectional view obtained by moving the transducer around the examined object and making the trace on the display follow a similar movement. Both A and B mode displays may be obtained with either simple or compound scanning. With simple scanning, the movement of the transducer is selected so that there is no superpositioning of lines of sight from the different directions. Linear and sector scanning are typical examples of simple scanning. With compound scanning, the movement of the transducer is selected so that there is superposition from different lines of sight. A combination of linear and sector scanning is one example of a compound scan.
If the reflecting surface (or interface) of interest is moving, its position may be plotted with time ("M mode") by using the B mode presentation and allowing the time base to be swept at right angles to its direction so as to display the movements of the interface echo backwards and forwards along the time base. This technique has been used to demonstrate the pulsatile movements of various parts of the heart and brain. If the B mode is used but the trace on the screen is made to represent the line of sight of the transducer, and then the transducer is scanned around the patient and the time base line on the screen made to follow, a two dimensional plot of impedance discontinuities is obtained. Two dimensional visualisation has been used in the pregnant uterus, abdomen, eye and breast.
Coupling from the transducer to the patient may be achieved by skin contact or by use of a water delay bath. If a water delay bath is used, the distance between the transducer and the skin surface must be greater than the largest depth of penetration to be used, to avoid ambiguity due to multiple reflections. In general, the skin contact scan results in greater comfort for the patient but echograms of less clarity, while the water delay scan gives less patient comfort and better quality echograms.
In order to compensate for the reduction in the energy of the ultrasonic pulse due to attenuation within the object under examination (for example tissue), the gain of the receiver is generally increased as the echo of the pulse is received from deeper reflecting surfaces within the object. This type of increase in gain is generally referred to as "time gain compensation" or "TGC". When using the echoscope equipment, the operator adjusts the sensitivity and slope of the TGC controlled amplifier after the first scan of a patient, then rescans the patient to obtain an image which is satisfactory for diagnosis. The gain controlled signals are then further processed and displayed in one of the ways described above.
In some receivers, TGC amplification is also followed by a non-linear compression amplification to further compress the size of the echoes so that they may be more readily displayed on the display unit. As the compression and display systems are non-linear, only qualitative information on echo size is displayed.
One deficiency of most examples of the apparatus that has hitherto been used in ultrasonic echoscopy is that the characteristics of the TGC control remain constant for the entire scan. This means that no account can be taken of local variation in tissue properties. This creates a problem because a local area, such as a bone or an air containing region, which is more highly absorbing than the surrounding tissue, casts an ultrasonic "shadow" which obscures deeper lying information.
One recent attempt to overcome the problem of variation in attenuation of signals within human tissue is described in the specification of U.S. Pat. No. 4,008,713 to J. M. Griffith and W. L. Henry. In the technique described in that specification, an amplifier of the received ultrasonic echo signals is switched rapidly from a high gain mode to a low gain mode to enable a particularly strong echo signal to be recognised from among a number of echo signals from the general region of the strong echo signal. This technique is shown to be applicable to echoscopy of the human heart, where it is important, when looking at the left ventricle, to be able to obtain information about the wall thicknesses of the cardiac structures, and to define accurately the location of the epicardial-lung interface. The septal and endocardial echoes are observed with a high gain amplification, while the epicardial signals are observed with low gain amplification. Such a technique is very limited in its application, and does not overcome the problem of "shadowing", which has been discussed above.
A system in which the gain of an amplification circuit is adjusted to compensate for various factors (absorption, "spreading", and interface reflection and scatter) is described in the specification of U.S. Pat. No. 4,043,181 to A. K. Nigam. One of the components of Nigam's amplification circuit is a time gain compensation unit, which is described in the passage from column 5, line 61 to column 6, line 22. It is clear from the description at column 6, lines 3 to 22 and column 8, lines 55 to 62, of specification No. 4,043,181 that Nigam's TGC unit involves the selection of a single attenuation factor, based on an average absorption value of the object under investigation. Thus Nigam's approach does not overcome the problem of "shadowing".
The specification of U.S. Pat. No. 4,057,049 to C. R. Hill does disclose a method of overcoming the problem of shadowing by adjusting the time gain compensation of the amplifier of received echo signals after determining the instantaneous attenuation values. However, the method used by Hill is rather complex, involving the measurement of the difference in values of the reflected echo signal at two frequencies. This difference is then used to automatically and instantaneously adjust the gain of the amplifier.